1. Technical Field
The present invention relates to an illumination device, a projector, and a polarization conversion element.
2. Related Art
In the past, a projector has been known as one of the devices capable of displaying a large-screen image. Projectors are provided with an illumination apparatus, an image formation apparatus, a projection lens, and so on. The illumination light emitted from the illumination apparatus is turned to image light representing the image by the image formation apparatus. The image light is enlargedly projected by the projection lens, and thus the large-screen image can more easily be obtained compared to direct-view image display devices.
As the illumination apparatus used for projectors or the like, there is known an apparatus provided with a lamp light source and a reflector. The reflector is formed of a part of a spheroid or a paraboloid having the focal point at the lamp light source. The light radially emitted from the lamp light source is reflected by the reflector, and proceeds in a direction along the illumination light axis. In general, the shape of the lamp light source and the shape of the reflector are arranged to be axisymmetric around the illumination light axis, and the light emitted from the illumination apparatus ideally has an axisymmetric intensity distribution.
Incidentally, enhancement of image quality and miniaturization of the apparatus are required for the projectors. As a technology effective for miniaturizing the apparatus for the projector, there can be cited a technology disclosed in JP-A-2003-16802 (a related art document). An illumination optical system (an illumination apparatus) of the related art document is provided with a light emitting section mainly composed of a first concave mirror, a second concave mirror, and a light source.
The light emitted from the light source toward the first concave mirror is reflected by the first concave mirror and then proceeds toward the second concave mirror. The light emitted from the light source toward the second concave mirror and the light reflected by the first concave mirror are emitted from the light emitting section. The light emitted from the light emitting section reaches an object to be illuminated, via an integrator for equalizing the density of the light beam, and so on.
For example, if it is arranged that the area to which the light is emitted from the light source is divided into two parts, and the second concave mirror reflects and folds the light emitted to one of the two parts, the cross-sectional area of the light beam emitted from the light emitting section becomes approximately a half thereof. Thus, the optical components disposed between the light emitting section and the object to be illuminated can be miniaturized, and the projector can also be miniaturized.
Although it is conceivable that the projector can be miniaturized using the technology of the related art document, there is a problem that the uniformity of the illuminance distribution on the illuminated object is degraded. This is because the light radially emitted from the light source is emitted from one of the two parts into which the area is divided, and thus the symmetry of the intensity distribution of the light emitted from the light source apparatus with respect to the boundary between the divisional parts is degraded. If the symmetry of the intensity distribution of the light emitted from the illumination apparatus is degraded, the uniformity of the illuminance distribution is degraded on the illuminated object, and in the case in which the illuminated object is an image forming apparatus, for example, since the image light formed fails to have a desired grayscale, the image quality is problematically degraded. Further, the light efficiency in the optical components disposed between the illumination apparatus and the illuminated object might be degraded.